The Senate is scheduled to vote on an immigration amendment introduced by Sen. Bob Corker, R-Tenn., left, and Sen. John Hoeven, R-N.D., last week. / Win McNamee, Getty Images

by Alan Gomez, USA TODAY

by Alan Gomez, USA TODAY

WASHINGTON - As the Senate prepares for a vote on Monday evening that all but assures final passage of a sweeping immigration bill that provides legal status to millions of undocumented immigrants, critics were putting on a last-minute, all-out push to stop it.

Republican senators opposed to the plan flooded the airwaves Sunday complaining about "fatal flaws" that remain in the bill. Conservative think tanks urged supporters to flood the Capitol switchboard with calls opposing the plan. And a group of 14 Republican senators sent a letter to Majority Leader Harry Reid, D-Nev., Monday morning complaining about his plan to limit amendments in order to pass the bill before the Senate leaves at the end of the week for a July 4 recess.

"This is deeply, deeply disturbing," the group wrote. "It is effectively shutting down the American people's ability to be heard on this issue through their elected representatives."

But supporters of the bill were moving forward Monday, inching closer to getting the 70 votes they believe is necessary to persuade the reluctant, Republican-led House of Representatives to vote on their bill.

Reid spokesman Adam Jentleson dismissed the letter from GOP senators as a "transparent attempt to suppress the strong bipartisan support for immigration reform." Sen. John Hoeven, R-N.D., and Sen. Bob Corker, R-Tenn., spent the weekend urging colleagues to support an amendment they brokered that adds nearly 20,000 Border Patrol agents to the Southwestern border and addresses other concerns raised by Republicans.

And the U.S. Chamber of Commerce launched a seven-figure ad buy on Monday to support the bill, and immigration advocacy groups were pushing their supporters to pressure their senators to keep the bill on track.

Hoeven and Corker reached their agreement with the bipartisan group of senators who wrote the bill, known as the Gang of Eight, on Friday. Critics, like Sen. Jeff Sessions, R-Ala., complained that they haven't had enough time to digest all of the changes made in the amendment, which was filed at just under 1,200 pages late Friday.

"We have an obligation to study it, read it and vote on the bill, not the talking points," Sessions said on the Senate floor Monday. "(Reid's) goal is to drive the train to passage by this Friday. Public policy? Public interest? Beside the point."

Hoeven said his amendment adds only 119 pages of new wording. "Very readable," he said Monday.

The immigration bill represents the biggest change to the nation's immigration laws since 1986. It would allow the nation's 11 million unauthorized immigrants to apply for U.S. citizenship, require all American business owners to use a federal database to verify the immigration status of new hires, add billions to secure the nation's border with Mexico and revamp the legal immigration system to bring in more high-tech and lower-skilled workers.

Rosemary Jenks, director of government relations for NumbersUSA, a group that opposes that bill, said the amendment brokered by Hoeven and Corker does little to help secure the border, and actually provides new loopholes that could weaken border security and allow future unauthorized immigrants to get U.S. citizenship.

She said the Senate passing the immigration bill is not the inevitability that many are making it seem.

"The American people still have the power to stop this," she said.

But Hoeven said he is confident the votes will be there, and that they have put together a responsible compromise that addresses most of the concerns from supporters and critics alike.

"When you have something that has this much impact, you need to get people on board in a bipartisan way to really make it work," he said. "We recognize that some people just aren't going to support the legislation. We understand and respect that. But we're trying to solve the problem we hope in a way that people feel works."

The Senate is scheduled to vote on the Hoeven-Corker amendment on Monday afternoon and then move quickly to vote on final passage within the week.